1. Field of the Invention
The present invention relates to an optical integrated device with ridge waveguide structure and a method of fabricating the same.
2. Description of the Related Art
In association with recent explosive increase of Internet users, there are now strong needs for the data transmission at an increasingly higher rate and with increasingly larger capacity, and it is generally considered that the optical communications will play an important role in the communication technology. Especially, in the optical communications for a long distance, the modulator-integrated type of light source, which has the modulator optically connected with the semiconductor laser, is used as an optical transceiver.
In addition, there are strong demands for a high value-added light source such as a tunable light source enabling instantaneous switching of wavelengths to satisfy the needs in the wavelength division multiplex transmission system. In the optical integrated device described above, aside from the active layer (laser section) for emitting optical signals, the different type multi-layer structure area such as a modulator section or a wavelength adjusting section are formed on same substrate. In the process in one of the methods for fabricating such an optical integrated device, the mask is formed on a multi-layer structure first grown, the unnecessary portions are removed by etching, and the different multi-layer structure is regrown. In the optical integrated device, the different type of multi-layer structure area is formed with the optical axis aligned for enabling the high optical coupling efficiency. In general, the regrow process described above is called the butt-joint (BJ).
There are two types of the optical device structures in general; the ridge waveguide structure and buried-hetero (BH) structure. Each of the two types includes advantages and disadvantages, respectively, as described below. In the ridge waveguide structure, the mesa etching is stopped above an active layer, for forming the mesa structure having the thickness of a few micrometers by etching an upper cladding layer. Thus, there is no damage caused to the active layer during mesa etching process. On the other hand, there is the disadvantage that the injected carrier is spread on over the active layer, causing increase of an loss current not contributing to the lasing operation. In the BH structure, the etching is performed on the layer below the active layer for forming the mesa structure, and then, both sides of the mesa structure are buried with the semiconductor layer. The carrier is, thus, efficiently injected into the active layer, and the threshold current is reduced as compared to the laser with ridge waveguide structure. On the other hand, there is the disadvantage in the BH structure that the active layer may be damaged during mesa etching. Especially for the active layer including aluminum, such as the active layer comprising of InGaAlAs materials, a specific treatment may be required for the side wall of active layer before the buried growth. The adoption of either structure may be allowable, depending on the device application. The optical device according to the present invention provides the improved optical device adopting the ridge waveguide structure.
The conventional technology is described below with reference to the modulator-integrated light source with ridge waveguide structure as an example. FIG. 1A is a perspective view illustrating a device. In FIG. 1, reference numeral 101 denotes an n-InP substrate; 102, a multiple quantum well (MQW1) layer in the laser section; 103, the waveguide(WG) layer in the waveguide section; 104, the MQW2 layer in the modulator section; 105, a p+-InGaAs contant layer; and 106, a p-InP cladding layer. A diffraction grating 107 is formed in the laser section. A optical window section and electrodes are omitted from FIG. 1 for simplification.
A semiconductor material such as InGaAsP or InGaAlAs is generally used as a material for the laser section or the modulator section. FIG. 1B is a cross-sectional view of a mesa A-A′ section. A diffraction grating is formed in the laser section, and the p-InP cladding layer 106 and the p+-InGaAs contact layer 105 are formed on the entire surface of the MQW layer and the WG layers that are connected to each other by BJ. FIG. 1C is a cross-sectional view of the mesa side B-B′ section. The mesa etching is stopped on the MQW layers 102 and 104 as well as on the WG layer 103 each having the high etching selectivity for InP. As described above, in the ridge waveguide structure type of devices, it is important to reduce a loss current for improving the device performance. Therefore, by stopping mesa etching just above the MQW layer, spread of an injected carrier is suppressed at minimum to enable lasing operation with a lower threshold current. In the final form of the device as described above, the BJ section is exposed in a portion of the device at the side region of mesa.
FIG. 2 is a view illustrating a flow of fabrication of the device shown in FIG. 1. At first, in the step (a), the MQW1 layer 203 in the laser section is grown on the n-InP substrate 201. In this step, the p-InP cap layer 202 is formed for protection of the surface in most cases. Then in step (b), a BJ mask1 204 is formed at a desired portion of a wafer. As a material of the mask, generally an insulating material such as SiO2 or SiN is used. In step (C), the p-InP cap layer 202 and the MQW 1 layer 203 are removed by using the BJ mask1 204 as an etching mask.
In step (d), the MQW2 layer 206 in the modulator section and the p-InP cap layer 205 are regrown. In this step, the MQW1 layer and the MQW2 layer are temporary butt-jointed. In step (e), after removing the mask1 204, the BJ mask2 207 are formed at desired positions of the laser section and the modulator section. In step (f), the p-InP cap layers 202 and 205, the MQW1 layer 203, and the MQW2 layer 206 are removed by etching using the BJ mask2 207 as an etching mask. Furthermore, in step (g), the WG layer 209 and the p-InP cap layer 208, were regrown. Then, the laser section, the waveguide section, and the modulator section are optically connected. In step (g), the WG section was butt-jointed to both the laser section and the modulator section concurrently. As described above, the laser section and the module section are temporary connected in step (d), and a transition region of a narrow band gap is formed near the mask because of the selective area growth effect in re-growing the MQW2 layer, which causes an optical absorption loss. The WG layer is, thus, introduced for removing the transition region. This is important for realizing a high performance of the optical device.
In step (h), after removing the mask 207 and forming a diffraction grating in the laser section, the p-InP cladding layer 211 and the p+-InGaAs contact layer 210 are grown in the whole area of the wafer. In step (i), the mesa mask 212 is formed. As the final step (j), the p+-InGaAs contact layer 210 and the p-InP cladding layer 211 are successively removed by etching in order to form a mesa stripe. The etching is stopped at the MQW1 layer, the MQW2 layer, and the WG layer comprising semiconductor materials including As. As a final form of the optical device, the surfaces of the BJ-connected portions are exposed at the portion in which the p-InP cladding layer is removed. Subsequently, by using the usual method, the device is completed by forming a passivated layer, and planarizing polyimide, and forming electrodes.